1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus, specifically to a reading optical system of an optical disk drive.
2. Description of the Related Art
FIG. 2 is a schematic drawing of a normal optical system of the optical disk drive. Laser light emitted from a semiconductor laser 1 becomes a collimated beam circular in cross section by a collimator 23 and a prism 2. Subsequently, the laser light is reflected by a polarizing beam splitter 31 and converted to circularly-polarized light by a quarter-wave plate 21. This circularly-polarized light is converged on an optical disk 50 by an objective lens 41. The optical disk 50 has a guiding trench in the interior of the disk 50 and a recorded mark whose reflectivity is different from that of a region without recording mark has been written. The length of the recording mark and the spacing therebetween are encoded according to information to be recorded. Since the optical disk 50 is rotating, the recording mark comes in and comes out of an irradiation position of the laser light and the quantity of reflected light varies with time.
The reflected light carrying information returns to the objective lens 41, and is converted from the circularly-polarized light to linearly-polarized light by the quarter-wave plate 21. Since this direction of polarization is orthogonal to a direction of polarization of the emitted light from the semiconductor laser 1, the light is transmitted by the polarizing beam splitter 31. This transmitted light is divided into two light beams by a beam splitter 32. The transmitted light of the beam splitter 32 is blocked into a half by a knife edge 22 and converged onto a split photodetector (a two-part split photodetector) 51 located at a focus position of a converging lens 42 by the converging lens 42.
When the optical disk 50 deviates in the direction of the optical axis and the focus of the irradiation light is not exactly on a layer bearing the recording marks thereon, the quantities of beams of the light falling on two light detecting elements of the split photodetector 51 become unbalanced. This unbalance is detected by an electronic circuit 60 as a differential signal, which is made to act as a focus error signal 71. The position of the objective lens 41 is adjusted by a lens actuator 63 using this focus error signal 71 so that the focus position of the emitted light of the objective light 41 is always on the layer bearing the recording marks, to avoid an out-of-focus state. On the other hand, the light reflected by the beam splitter 32 irradiates a split photodetector (a two-part split photodetector) 52 through the converging lens 43 with the light being out of focus to the photodetector 52. An electronic circuit 61 acts as a differential circuit, and an output therefrom becomes a tracking error signal 72. Moreover, the signals added up together by an electronic circuit 62 become a data signal 73.
In the conventional example of the optical disk drive described above, detection of the focus error signal is achieved by the knife edge method, and the tracking error signal is detected by a method called the diffracted light differential system. In addition to these, various methods have been proposed, and there are the astigmatic method, the image rotation method, etc. as typical detection methods of the focus error signal. Regarding detection methods of the tracking signal, there are the triplet-spot method, the wobbling method, and the phase difference method, etc. These methods have drawbacks as well as advantages with respect to optical alignment or efficiency of light, and hence are used properly according to the kind of drives. For these details, a book entitled, xe2x80x9cBasis and applications of optical disk storage,xe2x80x9d (supervised by Yoshito Tsunoda, edited by a corporate juridical person, Institute of Electronics, Information and Communication Engineers) gives detailed explanation.
At the moment when large-capacity and high-speed data transmission is being made to be possible, the optical disk as an information storage medium needs to be a high-density medium to record large-capacity data of moving picture information etc. To fulfill this task, there is no way except shortening the spacing of the recording marks and the track pitch of the optical information medium. Then, in order to read and write the optical information medium with a small spacing of the recording marks and the track pitch, a minute laser spot is prerequisite, and with this view the laser spot is being made minute down to a size comparable to a wavelength of the laser light with the use of an objective lens having a large numerical aperture (NA). However, there is a lower bound to the spot size that is governed by the diffraction limit, so it cannot be miniaturized interminably. Accordingly, the in-plane recording density that nominally depends on the size of the recording mark or the track pitch has a limit that is governed by a lower bound of the spot size. Then, it has been thought to try to increase the information density per unit area of the optical disk by adopting a layer configuration composed of more than one optical information recording layers.
When information is read from the multilayer optical disk, however, there occurs a crosstalk between the signals of the respective layers and a large error may be generated in the read signal with the conventional type optical head. One of reasons for such malfunction is that the focus position of the laser light deviates away from an optical information medium layer that is intended to be read.
The technology that eliminates interference in the focus error signal is disclosed by JP-A-222867/1998 entitled xe2x80x9cOptical Pick-Up Apparatus.xe2x80x9d This technology is to provide auxiliary light-receiving regions having small areas on both sides of the split photodetector. This technology stands on a basic idea that a spread of the focus error signal for a single layer is narrowed, but an incident state of the reflected light from a neighboring optical information medium layer on the photodetector is not considered. Therefore, it is necessary to correct electrically the quantities of the signals from the auxiliary light-receiving regions to generate the focus error signal.
The present invention aims at reducing the crosstalk of the focus error signal originated from the neighboring optical information medium layer in a read optical system of the optical disk drive for reading the multilayer optical disk.
FIG. 3 is a schematic diagram for illustrating the problem of the focus error signal generated from the multilayer disk. FIG. 3 shows a schematic cross section of a two-layer optical disk 501 having a first optical information medium layer 511 and a second optical information medium layer 512 therein. Irradiation light 80 from a semiconductor laser (not shown in the figure) is converged by the objective lens 41 and is in focus to the first optical information medium layer 511. Reflected light therefrom is transmitted through the objective lens 41 to become a collimated beam and a part of the collimated beam is blocked by the knife edge 22 on the way. The other part of the light that avoids being blocked by the knife edge 22 falls on the split photodetector 51 through the converging lens 43, as being in a state 81.
As shown in FIG. 4, the reflected light 81 from the first optical information medium layer 511 is converged to be a small spot on a split position of the split photodetector 51. In this state, the same quantity of light falls on each of the light detecting elements of the split photodetector 51, respectively. Since the focus error signal is obtained as a differential signal of the split photodetector, if the light beam is in focus to the optical information medium layer, the focus error signal becomes zero because of a zero differential signal. In the case of a single-layer optical disk having no neighboring layer, the objective lens can be controlled by a servomechanism on the basis of this signal in order that the focus position becomes optimal. However, if there is the neighboring layer, the reflected light by the neighboring layer affects the focus error signal adversely. The reason will be described as follows.
Since the optical disk 501 has a structure of the multilayer, the laser light 82 having passed through the first optical information medium layer 511 also irradiates the second optical information medium layer 512 while the laser light 82 is out of focus, as shown in FIG. 3. Reflected light 83 from the second optical information medium layer 512 after passing through the objective lens 41 becomes not a collimated beam but a converging beam. A position at which a beam size of the reflected light 84 becomes minimum after passing through a converging lens 43 gets closer to the converging lens 43, and comes into a state of being extended on a detection plane of the split photodetector 51. FIG. 5 shows an irradiation state of the reflected light 84 from the second information medium layer 512, on the split photodetector 51. The reflected light 84 falls on only one of the light detecting elements of the split photodetector 51.
In the foregoing, the reflected light from the first optical information medium layer 511 and that from the second optical information medium layer 512 are described separately with respect to the incidence state to the split photodetector 51, but practically these beams of the reflected light fall on the split photodetector 51 simultaneously. Therefore, although the light is in focus to the first optical information medium layer 511, the differential signal from the split photodetector 51 becomes a non-zero value, overall light signals being unbalanced. If the objective lens 41 gets cornrolled by a servomechanism in this state, the optical disk deviates from the focus position. This phenomenon also occurs in the case where the second optical information medium layer is located on the objective lens 41 side of the first optical information medium layer, the optical outputs become unbalanced in a direction opposite to the above case.
FIG. 6 shows the differential signal (focus error signal) when the optical disk 501 is being moved away from the objective lens 41. The horizontal axis of FIG. 6 indicates the focus position of the objective lens 41. In the figure, a point 74 and a point 75 roughly indicates positions of the second and first optical information medium layers 512, 511, respectively, and positions at which the differential signal becomes zero do not agree with the positions of the medium layers 74 and 75. An object of the present invention is to reduce the effect of the reflected light from the other medium layer that appears in the focus error signal, in consideration of the problem when reading the multilayer optical disk.
In this invention, a part of the photodetector on which the reflected light from the neighboring layer falls is divided into light detecting elements and a difference of the signals from the photodetector thus divided are taken to be canceled out, whereby the effect of the reflected light from the neighboring layer is avoided.
An optical information recording apparatus of the present invention records information by irradiating the optical disk having plurality of optical information medium layers therein with light, wherein it comprises:
a light source;
light irradiation means for converging the light from the light source and irradiating the optical disk therewith;
light converging means for extracting a part of returned light from the optical disk that passes a predetermined spatial region and converging it; and
a photodetector for detecting the returned light from the optical disk that is converged by the light converging means. The photodetector further comprises:
first and second light detecting elements arranged such that, when one of the optical information medium layers of the optical disk is irradiated by the light irradiation means with the focus position of the light irradiation means being thereon, outputs of the first and second light detecting elements caused by the returned light from the one optical information medium layer are approximately equal to each other;
a third light detecting element arranged such that its output caused by the returned light from an optical information medium layer other than the one optical information medium layer is approximately equal to that of the first light detecting element; and
a fourth light detecting element arranged such that its output caused by the returned light from an optical information medium layer other than the one optical information medium layer is approximately equal to that of the second light detecting element.
An optical information reproducing apparatus of the present invention reproduces information from the optical disk having the first and second optical information medium layers therein, wherein it comprises:
a light source;
light irradiation means for converging the light from the light source and irradiating the optical disk therewith;
light converging means for extracting a part of the returned light from the optical disk that passes a predetermined spatial region and converging it; and
a photodetector for detecting the returned light from the optical disk that is converged by the light converging means.
The photodetector further comprises:
first and second light detecting elements arranged such that, when either of the first or second optical information medium layer of the optical disk is irradiated by the light irradiation means with the focus position of the light irradiation means being thereon, outputs of the first and second light detecting elements caused by the returned light from the one optical information medium layer are approximately equal to each other;
a third light detecting element arranged such that, when the first optical information medium layer of the optical disk is irradiated by the light irradiation means with the focus position of the light irradiation means being thereon, its output caused by the returned light from the second optical information medium layer is approximately equal to that of the first light detecting element; and
a fourth light detecting element arranged such that, when the second optical information medium layer of the optical disk is irradiated by the light irradiation means with the focus position of the light irradiation means being thereon, its output caused by the returned light from the first optical information medium layer is approximately equal to that of the second light detecting element.
In the optical information recording/reproducing apparatus, the focus error signal can be generated on the basis of a difference between a sum of the outputs of the first and fourth light detecting elements and a sum of the outputs of the second and third light detecting elements.
Preferably, light receiving areas of the first and second light detecting elements are set to be smaller than the light receiving areas of the third and fourth light detecting elements.
The optical head apparatus of the present invention further comprises:
a light source;
light irradiation means for converging the light from the light source and irradiating the optical disk therewith; driving means for adjusting the focus position of the light irradiation means;
light converging means for extracting a part of the returned light from the optical disk that passes a predetermined spatial region and converging it;
a photodetector for detecting the returned light from the optical disk that is converged by the light converging means; and
control means for generating the focus error signal from the outputs of the photodetector and performing feedback control of the driving means using the said focus error signal. In the optical head apparatus, there is provided, as the photodetector, a four-part split photodetector consisting of two detecting elements in the inner locations and two detecting elements in the outer locations, wherein
the control means generates the focus error signal by adding the signals of the detecting elements of the photodetector with alternating polarities given to the respective signals in such a way that any adjacent elements have mutually different polarities.
According to the present invention, in the optical disk having multiple optical information medium layers therein, precise control of the focus position can be performed and hence reliability in reading the multilayer optical disk is improved.
Other and further objects, features and advantages of the invention will appear more fully from the following description.